Article including a substrate with a metallic coating and a protective coating thereon, and its preparation and use in component restoration

ABSTRACT

A coated article has a metallic substrate with a substrate composition, and a metallic coating overlying and contacting the metallic substrate. The metallic coating has a metallic-coating composition different from the substrate composition. A protective coating overlies and contacts the metallic coating. The protective coating includes an aluminide layer overlying and contacting the metallic coating, and optionally a thermal barrier coating overlying and contacting the aluminide layer. This structure may be used to restore a key dimension of an article that has previously been in service and to protect the article as well.

[0001] This invention relates to the protection of a substrate using aprotective coating and, more particularly, to the restoration of a keydimension of an article such as a gas turbine component that haspreviously been in service.

BACKGROUND OF THE INVENTION

[0002] In an aircraft gas turbine (jet) engine, air is drawn into thefront of the engine, compressed by a shaft-mounted compressor, and mixedwith fuel. The mixture is burned, and the hot combustion gases arepassed through a turbine section mounted on the same shaft. In theturbine section, the hot combustion-gas flow passes between pairs ofturbine vanes (also sometimes termed the “nozzles”), which redirect thecombustion-gas flow slightly, and impinges upon the turbine blades. Theimpingement of the flow of hot combustion gas against an airfoil sectionof the turbine blades turns the shaft and provides power to thecompressor. The hot exhaust gases flow from the back of the engine,driving it and the aircraft forward.

[0003] The hotter the combustion and exhaust gases, the more efficientis the operation of the jet engine. There is thus an incentive to raisethe combustion and exhaust gas temperatures. However, at these hightemperatures the combustion-gas flow is highly corrosive, erosive, andoxidative to the materials it contacts. The maximum temperature of thecombustion gases is normally limited by the materials used to fabricatethe turbine components of the engine, including the turbine blades andvanes. The turbine components must have the necessary strength, but alsobe resistant to the environmental damage caused by the combustion-gasflow, at the operating temperature. In current engines, the turbinevanes and blades are made of cobalt alloys and nickel-based superalloys,and can operate at temperatures of up to about 1800-2100° F. Thesecomponents are subject to environmental damage by corrosion, erosion,and oxidation at those temperatures.

[0004] Many approaches have been used to increase the operatingtemperature limits and service lives of the turbine blades and vanes totheir current levels, while achieving acceptable environmentalresistance. The composition and processing of the base materialsthemselves have been improved. Cooling techniques are used, as forexample by providing the component with internal cooling passagesthrough which cooling air is flowed.

[0005] In another approach used to protect the turbine-sectioncomponents, a portion of the surfaces of the turbine blades or vanes iscoated with a protective coating. One type of protective coatingincludes an aluminum-containing protective coating deposited upon thesubstrate material to be protected. The exposed surface of thealuminum-containing protective coating oxidizes to produce an aluminumoxide protective scale that protects the underlying surface. A ceramicthermal barrier coating may be applied over the aluminum-containingprotective coating to further protect and insulate the substrate.

[0006] Despite careful selection of the base materials and protectivecoating, after a gas turbine component has been in service, it isusually eroded, corroded, and oxidized so that one or more keydimensions of the component may be reduced below respective minimumpermissible dimensional values. An example relates to the throatseparation dimension between each adjacent pair of gas turbine vanes,the space through which the hot combustion-gas flows from the combustoron its way to contact the turbine blades. The throat dimension betweentwo adjacent gas turbine vanes has its maximum permissible dimensionalvalue that cannot be exceeded without reducing the performance andefficiency of the gas turbine. When the gas turbine vanes are operatedin service, their surfaces are worn away so that a key thicknessdimension of the vanes become dimensionally smaller. Consequently, thevane-to-vane gas-flow throat-separation dimension becomes larger and themaximum permissible throat-separation dimensional value is eventuallyexceeded as the key thickness dimension of the vanes falls below itsminimum permitted key thickness dimension.

[0007] The gas turbine components are expensive to fabricate, and it istherefore desirable, where feasible and the service damage is not toogreat, to repair and restore them, rather than to discard them. Norestoration procedure has been proposed for these protected componentsto restore dimensions and also to restore the protective structure, andtherefore a need exists for such a restoration procedure. The presentinvention fulfills this need, and further provides related advantages.

SUMMARY OF THE INVENTION

[0008] The present approach provides an article that is protectedagainst environmental damage, and is also dimensionally controlled. Theapproach may be used to restore dimensions of articles that havepreviously been in service, and whose dimensions are reduced belowminimum permissible dimensional values during that service. It may alsobe used with newly made articles if their dimensions must be increased.

[0009] A coated article comprises a metallic substrate having asubstrate composition and a metallic coating overlying and contactingthe metallic substrate. The metallic coating has a metallic-coatingcomposition different from the substrate composition. Themetallic-coating composition must be readily and controllablydepositable in thin layers, such as those in the range of from about0.003 to about 0.015 inch, but at a rate sufficiently high to beeconomically feasible. A protective coating overlies and contacts themetallic coating. The protective coating comprises an aluminide layeroverlying and contacting the metallic coating and, optionally, a ceramicthermal barrier coating overlying and contacting the aluminide layer.

[0010] In a case of interest, the substrate is a component of a gasturbine engine, such as a turbine blade or turbine vane (nozzle). Thesubstrate composition is preferably either a nickel-base alloy, such asa nickel-base superalloy, or a cobalt-base alloy. In an application ofmost interest, the substrate is a component of a gas turbine engine thathas previously been in service without the metallic coating thereon.

[0011] Where a key dimension of the article has been reduced below itsminimum permissible dimensional value during service as a result of theremoval of substrate material due to environmental factors, the keydimension cannot be restored simply by making the protective coatingthicker during the repair procedure, in many cases. The thicknesses ofthe layer or layers of the protective coating are optimized to providethe necessary protection, stability, and resistance to removal duringsubsequent service. Specifically, if the aluminide layer and/or theceramic thermal barrier coating are made overly thick, there is atendency for them to spall away as a result of thermally inducedstresses and other reasons. Additionally, the protective coating doesnot have the same mechanical properties as the substrate material, sothat replacement of the removed substrate material with the material ofthe protective coating results in a net weakening of the article.

[0012] In applications of the present approach, the metallic coating isapplied to the substrate to build up the thickness of the substrate,before the protective coating is applied. The metallic coating has acomposition different from that of the substrate. The metallic coatingis selected both for having good mechanical properties that are similarto those of the substrate material, but also for its ability to bedeposited in a moderately thick layer. In a typical example, themetallic coating has a thickness of at least about 0.003 inch,preferably from about 0.003 to about 0.010 inch, but in some cases asthick as 0.015 inch or more. The metallic coating restores the thicknessof the substrate back to about its original thickness, less thethickness of the protective coating, and then the optimal protectivecoating is applied over the metallic coating to restore the requiredoverall key dimension.

[0013] The metallic coating is not a diffusion aluminide protectivecoating and is not an overlay aluminide protective coating. Thediffusion aluminide protective coating and the overlay aluminideprotective coating have a high aluminum content, typically above about16 percent by weight. Such high-aluminum coatings give good oxidationprotection, but they do not have mechanical properties comparable withthe substrate material. In the case where the metallic coating is anickel-base alloy, and preferably a nickel-base superalloy, the aluminumcontent is typically less than about 10 percent by weight, and there isa high content of other elements found in superalloys for strength andother properties, such as tantalum, tungsten, molybdenum, chromium,rhenium, zirconium, titanium, niobium, boron, and/or carbon.

[0014] The aluminide layer is desirably a platinum-group metal-aluminidelayer. That is, the aluminide is formed by interdiffusing an appliedaluminum sublayer with an applied sublayer of platinum, palladium, orrhodium. The aluminide layer may also be formed by interdiffusing anapplied aluminum sublayer with an applied sublayer of another elementsuch as chromium. The aluminide layer may instead be a simple aluminide.

[0015] A method for restoring a key dimension of and protecting anarticle comprises first providing an article that has previously been inservice, wherein the key dimension of the article is below a minimumpermissible dimensional value. The article serves as a substrate havinga substrate composition. A metallic coating is deposited overlying andcontacting the metallic substrate, wherein the metallic coating has ametallic-coating composition different from the substrate composition. Aprotective coating is applied overlying and contacting the metalliccoating, wherein the metallic coating and the protective coatingtogether have a restoration thickness sufficient to increase the keydimension to no less than the minimum permissible dimensional value. Theprotective coating comprises an aluminide layer overlying and contactingthe metallic coating. Compatible features discussed herein may be usedwith this method.

[0016] Other features and advantages of the present invention will beapparent from the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings, whichillustrate, by way of example, the principles of the invention. Thescope of the invention is not, however, limited to this preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a perspective view of a gas turbine vane airfoil;

[0018]FIG. 2 is a schematic plan view of three gas turbine vanes,forming two pairs of vanes;

[0019]FIG. 3 is an enlarged sectional view of one of the gas turbinevanes of FIG. 2, taken generally on line 3-3; and

[0020]FIG. 4 is a block flow diagram of a preferred approach forpracticing the present approach.

DETAILED DESCRIPTION OF THE INVENTION

[0021]FIG. 1 depicts a portion of an article 20 that is an embodiment ofthe present approach and may be prepared by the present approach. Inthis case, the article 20 is a component of a gas turbine engine, andspecifically a gas turbine vane 22 (also called a “nozzle”). The gasturbine vane 22 has the illustrated airfoil 24 and is attached to itssupport by end structure (not shown). The present approach is notlimited to a turbine vane, which is the preferred application but whichis presented as an example.

[0022]FIG. 2 depicts three of the turbine vanes 22 arranged relative toeach other as they are in a gas turbine engine. The turbine vanes 22 maybe described as being two adjacent pairs of turbine vanes. A throatseparation dimension D_(O) is the distance of closest approach betweenthe two vanes of each pair. The throat area TA is the product of thethroat width D_(O) times the length L of the airfoil measured parallelto the lengthwise direction of the airfoil 24 (see FIG. 1). To maintainthe throat area TA within its required tolerances, D_(O) must be keptwithin its required tolerances. During service, the thickness t_(V) ofthe turbine vanes 22 tends to decrease due to environmental damage, sothat D_(O) increases. However, if D_(O) becomes too large and exceedsits maximum permitted throat separation dimension, the operation andefficiency of the gas turbine engine unacceptably decreases. Thus, thethickness t_(V) must be equal to or greater than the minimum permissibledimensional value t_(V,MIN). The situation wherein t_(V) becomes lessthan t_(V,MIN) usually arises after the turbine vane 22 is operated inservice, but it may arise in new-make articles are well.

[0023]FIG. 3 depicts the surface region of the turbine vane 22 ingreater detail. The body of the turbine vane 22 serves as a substrate 26having a substrate composition. The substrate composition is preferablythat of a nickel-base alloy such as a nickel-base superalloy, or acobalt-base alloy. An “X-base” alloy is an alloy having more of elementX by weight than any other element. A nickel-base superalloy is anickel-base alloy that is strengthened by the precipitation of gammaprime or a related phase. A cobalt-base alloy has more cobalt by weightthan any other element, and a cobalt-base superalloy typically isstrengthened by the presence of large atoms that provide solid solutionstrengthening and by the precipitation of carbide phases. An example ofa cobalt-base substrate composition of most interest is MAR M-509,having a nominal composition, in weight percent, of about 0.6 percentcarbon, about 0.1 percent manganese, about 0.4 percent silicon, about22.5 percent chromium, about 0.2 percent titanium, about 1.5 percentiron, about 0.01 percent boron, about 0.5 percent zirconium, about 10percent nickel, about 7 percent tungsten, about 3.5 percent tantalum,balance cobalt and minor elements. The present approach is not limitedto this alloy, which is presented as an example.

[0024] A metallic coating 28 overlies and contacts the metallicsubstrate 26. The metallic coating 28 has a metallic-coating compositiondifferent from the substrate composition. The metallic coating 28 has acomposition different from that of the substrate that may be readilydeposited onto the substrate 26. Most substrate compositions simplycannot be readily deposited as thin layers by readily available andeconomically feasible deposition techniques, such as those that may beused for the metallic-coating composition. The metallic coating 28 ismetallic in nature, and is not a diffusion aluminide protective coatingand is not an overlay aluminide protective coating. The diffusionaluminide protective coating and the overlay aluminide protectivecoating have a high aluminum content, typically above about 16 percentby weight. Such high-aluminum coatings give good oxidation protection,but they do not have mechanical properties comparable with the substratecomposition. In the case where the metallic coating 28 is a nickel-basealloy, and preferably a nickel-base superalloy, the aluminum content istypically less than about 10 percent by weight, and there is a highcontent of other elements found in superalloys for strength and otherproperties, such as tantalum, tungsten, molybdenum, chromium, rhenium,zirconium, titanium, niobium, boron, and/or carbon. A preferred metalliccoating 28 is alloy BC-52, having a nominal composition, in weightpercent, of about 18 percent chromium, about 6.5 percent aluminum, about10 percent cobalt, about 6 percent tantalum, about 2 percent rhenium,about 0.5 percent hafnium, about 0.3 percent yttrium, about 1 percentsilicon, about 0.015 percent zirconium, about 0.015 percent boron, about0.06 percent carbon, the balance nickel and incidental impurities. Thepresent approach is not limited to this metallic coating 28, however.

[0025] A protective coating 30 overlies and contacts the metalliccoating 28. The protective coating 30 includes at least an aluminidelayer 32 overlying and contacting the metallic coating 28. The aluminidelayer 32 is preferably a platinum-group metal-aluminide layer. Theplatinum-group metal aluminide comprises interdiffused aluminum and aplatinum-group metal, preferably platinum, palladium, and/or rhodium,and most preferably platinum. The aluminide layer 32 partiallyinterdiffuses with the metallic coating 28 after exposure at elevatedtemperature. The aluminide layer 32 preferably has at least about 16percent by weight aluminum and 16 percent by weight of theplatinum-group metal.

[0026] After exposure to oxygen at elevated temperature, an outermostportion of the aluminide layer 32 oxidizes to produce an aluminum oxide(alumina) scale 34. The aluminum oxide scale 34 resists furtheroxidation, protecting the underlying structure. For the presentpurposes, the aluminum oxide scale 34 is considered part of thealuminide layer 32.

[0027] Optionally, a ceramic thermal barrier coating 36 is part of theprotective coating 30 and overlies and contacts the aluminide layer 32.The thermal barrier coating 36 is preferably zirconia having from about3 to about 12 percent yttria therein, a ceramic termed yttria-stabilizedzirconia, or YSZ.

[0028] The aluminide layer 32 preferably has a thickness of from about0.0005 to about 0.004 inch thick. The ceramic thermal barrier coating36, where present, preferably has a thickness of from about 0.005 toabout 0.020 inch, more preferably from about 0.005 to about 0.015 inch.These thicknesses are selected because of the requirement for protectionof the underlying structure, and also so that the respective thicknessesare not so great that the aluminide layer 32 and ceramic thermal barriercoating 36 debond from their respective underlying structures. Thesethicknesses cannot be substantially increased to replace metal lost orabsent from the underlying substrate 26. Instead, the metallic coating28 replaces the metal lost or absent from the substrate 26, to bring thedimension t_(V) back to above the minimum permissible dimensional valuet_(V,MIN). The metallic coating 28 has strength properties generallysimilar to those of the substrate 26, so that replacement of the lostmetal of the substrate 26 with the metallic coating 28 does not resultin a substantial loss of overall strength properties of the article 20.The materials of the protective coating 30, on the other hand, are muchweaker than the material of the substrate 26. In a typical case, themetallic coating 28 has a thickness of at least about 0.003 inch and insome cases as high as about 0.015 inch, and usually has a thickness offrom about 0.005 to about 0.010 inch.

[0029]FIG. 4 depicts a preferred method of practicing the invention torestore a key dimension such as t_(V) and to protect the article 20. Themetallic article 20 or 22 that has previously been in service isprovided, step 50. The metallic article is preferably made of acobalt-base alloy or a nickel-base alloy. The key dimension t_(V) of thearticle 20 is below the minimum permissible dimensional value t_(V,MIN).The key dimension may instead be below the minimum permissibledimensional value in a new-make article, but the situation most oftenarises in a repair context. The article 20 or 22 serves as the metallicsubstrate 26 having the substrate composition.

[0030] The metallic coating 28 is deposited overlying and contacting themetallic substrate 26, step 52. The metallic coating 28 has itsmetallic-coating composition different from the substrate composition.In the preferred case, the BC-52 nickel-base superalloy metallic coatingis applied by any operable approach, but is preferably applied by theHVOF (high-velocity oxyfuel) process or the LPPS (low-pressure-plasmaspray) process, both of which are deposition processes that are known inthe art. The HVOF process, which utilizes a high velocity gas as aprotective shield to prevent oxide formation, is a relatively lowtemperature thermal spray that allows for application of a high densityoxide-free coating in a wide variety of thicknesses, is known in theart. The HVOF process typically uses any one of a variety of fuel gases,such as oxypropylene, oxygen/hydrogen mixtures or kerosene. Gas flow ofthe fuel can be varied from 2000-5000 ft/sec. The temperature of thespray will depend on the combustion temperature of the fuel gas used,but will typically be in the range of 3000-5000° F. The LPPS processapplies the coating as a plasma spray at a reduced deposition pressure.

[0031] The protective coating 30 is deposited overlying and contactingthe metallic coating, step 54. The metallic coating 28 and theprotective coating 30 together have a restoration thickness sufficientto increase the key dimension t_(V) to no less than the minimumpermissible dimensional value t_(V,MIN).

[0032] To deposit the aluminide layer in the preferred form of aplatinum aluminide, a platinum-containing layer is first deposited ontothe surface of the metallic coating 28. The platinum-containing layer ispreferably deposited by electrodeposition. For the preferred platinumdeposition, the deposition is accomplished by placing aplatinum-containing solution into a deposition tank and depositingplatinum from the solution onto the surface of the metallic coating 28.An operable platinum-containing aqueous solution is Pt(NH₃)₄HPO₄ havinga concentration of about 4-20 grams per liter of platinum, and thevoltage/current source is operated at about ½-10 amperes per square footof facing article surface. The platinum-containing layer, which ispreferably from about 1 to about 6 micrometers thick and most preferablyabout 5 micrometers thick, is deposited in 1-4 hours at a temperature of190-200° F.

[0033] A layer comprising aluminum and any modifying elements isdeposited over the platinum-containing layer by any operable approach,with chemical vapor deposition preferred. In that approach, a hydrogenhalide activator gas, such as hydrogen chloride, is contacted withaluminum metal or an aluminum alloy to form the corresponding aluminumhalide gas. Halides of any modifying elements are formed by the sametechnique. The aluminum halide (or mixture of aluminum halide and halideof the modifying element, if any) contacts the platinum-containing layerthat overlies the metallic coating 28, depositing the aluminum thereon.The deposition occurs at elevated temperature such as from about 1825°F. to about 1975° F. so that the deposited aluminum atoms interdiffusewith the platinum-containing layer and somewhat with the uppermostportion of the metallic coating 28 during a 4 to 20 hour cycle.

[0034] Where used, the ceramic thermal barrier coating 36 is deposited,preferably by a physical vapor deposition process such as electron beamphysical vapor deposition (EBPVD). The ceramic thermal barrier coating36 is preferably from about 0.005 to about 0.020 inch thick. The ceramicthermal barrier coating 42 is preferably yttria-stabilized zirconia(YSZ), which is zirconium oxide containing from about 3 to about 12weight percent, preferably from about 6 to about 8 weight percent, ofyttrium oxide. Other operable ceramic materials may be used as well.Examples include yttria-stabilized zirconia which has been modified withadditions of “third” oxides such as lanthanum oxide, ytterbium oxide,gadolinium oxide, cerium oxide, neodymium oxide, tantalum oxide, ormixtures of these oxides, which are co-deposited with the YSZ.

[0035] The present invention has been reduced to practice. Nozzlesegments that have been previously been in service have been repaired bythe approach discussed above on a prototype-process basis. The dimensiont_(V) was initially below the specification limit, and was increased towithin the specification limit by the present approach.

[0036] Although a particular embodiment of the invention has beendescribed in detail for purposes of illustration, various modificationsand enhancements may be made without departing from the spirit and scopeof the invention. Accordingly, the invention is not to be limited exceptas by the appended claims.

What is claimed is:
 1. A coated article comprising: a metallic substratehaving a substrate composition; a metallic coating overlying andcontacting the metallic substrate, wherein the metallic coating has ametallic-coating composition different from the substrate composition;and a protective coating overlying and contacting the metallic coating,wherein the protective coating comprises an aluminide layer overlyingand contacting the metallic coating.
 2. The article of claim 1, whereinthe substrate composition is selected from the group consisting of anickel-base alloy and a cobalt-base alloy.
 3. The article of claim 1,wherein the substrate is a component of a gas turbine engine.
 4. Thearticle of claim 1, wherein the substrate is a component of a gasturbine engine that has previously been in service without the metalliccoating thereon.
 5. The article of claim 1, wherein the metallic coatingis a nickel-base superalloy.
 6. The article of claim 1, wherein themetallic coating has a thickness of at least about 0.003 inch.
 7. Thearticle of claim 1, wherein the aluminide layer is a platinum-groupmetal-aluminide layer.
 8. The article of claim 1, wherein the protectivecoating further comprises a ceramic thermal barrier coating overlyingand contacting the aluminide layer.
 9. A coated article comprising: ametallic substrate that is a component of a gas turbine engine and has asubstrate composition selected from the group consisting of anickel-base alloy and a cobalt-base alloy, wherein the substrate is acomponent of a gas turbine engine that has previously been in servicewithout the metallic coating thereon; a metallic coating overlying andcontacting the metallic substrate, wherein the metallic coating has ametallic-coating composition different from the substrate composition;and a protective coating overlying and contacting the metallic coating,wherein the protective coating comprises a platinum-groupmetal-aluminide layer overlying and contacting the metallic coating. 10.The article of claim 9, wherein the metallic coating is a nickel-basesuperalloy.
 11. The article of claim 9, wherein the metallic coating hasa thickness of at least about 0.003 inch.
 12. The article of claim 9,wherein the protective coating further comprises a ceramic thermalbarrier coating overlying and contacting the aluminide layer.
 13. Amethod for restoring a key dimension of and protecting an article,comprising the steps of providing an article that has previously been inservice, wherein the key dimension of the article is below a minimumpermissible dimensional value, and wherein the article serves as ametallic substrate having a substrate composition; depositing a metalliccoating overlying and contacting the metallic substrate, wherein themetallic coating has a metallic-coating composition different from thesubstrate composition; depositing a protective coating overlying andcontacting the metallic coating, wherein the metallic coating and theprotective coating together have a restoration thickness sufficient toincrease the key dimension to no less than the minimum permissibledimensional value, and wherein the protective coating comprises analuminide layer overlying and contacting the metallic coating.
 14. Themethod of claim 13, wherein the step of providing the article includesthe step of providing the article having the substrate compositionselected from the group consisting of a nickel-base alloy and acobalt-base alloy.
 15. The method of claim 13, wherein the step ofproviding the article includes the step of providing the article that isa component of a gas turbine engine.
 16. The method of claim 13, whereinthe step of providing the article includes the step of providing thearticle that is a gas turbine vane.
 17. The method of claim 13, whereinthe step of depositing the metallic coating includes the step ofdepositing the metallic coating that is a nickel-base superalloy. 18.The method of claim 13, wherein the step of depositing the metalliccoating includes the step of depositing the metallic coating having athickness of at least about 0.003.
 19. The method of claim 13, whereinthe step of depositing the protective coating includes the step ofdepositing the aluminide layer as a platinum-group metal-aluminidelayer.
 20. The method of claim 13, wherein the step of depositing theprotective coating includes the step of depositing a ceramic thermalbarrier coating overlying and contacting the aluminide layer.